A structural material for use in turbochargers, jet engines, gas turbines, space planes or the like, which is to be exposed to high-temperature atmospheres, includes heat-resistant Ti alloys, such as TiAl based intermetallic compounds [Ti3Al (α2 phase) and TiAl (γ phase)] and high-temperature titanium alloys [α+β type: Ti-6Al-4V alloy, Ti-6Al-4Mo-4Cr (incl. Zn, Sn) alloy; near α type: Ti-6Al-4Zr-2.8Sn alloy; near β type: Ti-5Al-3Mo-3Cr-4Zr-2Sn alloy]; superalloys, such as Ni-based, Co-based and Fe-based heat-resistant alloys; other heat-resistant alloys, such as Nb-based, Ir-based and Re-based heat-resistant alloys; carbon materials; and other various intermetallic compounds.
It is often the case that a high-temperature atmosphere in contact with the heat-resistant alloy material contains an oxidative or corrosive substance, such as oxygen or water vapor. If the heat-resistant alloy material is exposed to a corrosive high-temperature atmosphere, the reaction between the alloy material and the corrosive substance in the atmosphere will be liable to cause accelerated oxidation and/or high-temperature corrosion in the alloy material. It is also likely that O, N, S, Cl and/or C diffusing from the atmosphere into the heat-resistant alloy material causes internal corrosion in the surface of the heat-resistant alloy material, which leads to deterioration in material strength.
Such high-temperature corrosion can be prevented by covering the surface of the heat-resistant alloy material with a protective film excellent in environment blocking performance. The protective film is typically made of Al2O3, SiO2 or Cr2O3, and formed by diffusing Al, Si or Cr from a base to a surface layer of a heat-resistant alloy material in an oxidizing atmosphere (see, for example, the following Patent Publications 1 to 3 and Non-Patent Publication 1) or by depositing Al2O3, SiO2 or Cr2O3 on the surface of a heat-resistant alloy material through a CVD process, a thermal spraying process, a reactive sputtering process or the like. The Al2O3, SiO2 or Cr2O3 film can suppress the reaction between the oxidative substance in the atmosphere and the metal elements of the heat-resistant alloy material to maintain excellent high-temperature characteristics inherent in the heat-resistant alloy.
Patent Publication 1: Japanese Patent Laid-Open Publication No. 05-156423 (U.S. Pat. No. 2,948,004)
Patent Publication 2: Japanese Patent Laid-Open Publication No. 06-093412 (U.S. Pat. No. 2,922,346)
Patent Publication 3: Japanese Patent Laid-Open Publication No. 09-324256
Non-Patent Publication 1: C. Zhou, H. Xu, S. Gong, Y Yang and K.-Y. Kim; Surface and Coating Technology 132 (2000), p. 117.